Diffusion transfer products and processes have been the subject of several patents, including for example, U.S. Pat. Nos. 2,983,606; 3,345,163; 3,362,819; 3,594,164; and 3,594,165.
In general, diffusion transfer photographic products and processes involve film units having a photosensitive system including at least one silver halide layer, usually integrated with an image-providing material, e.g., an image dye-providing material. After photoexposure, the photosensitive system is developed, generally by uniformly distributing an aqueous alkaline processing composition over the photoexposed element, to establish a diffusible imagewise distribution of the image-providing material. The image-providing material is selectively transferred, at least in part, by diffusion to an image-receiving layer or element positioned in a superposed relationship with the developed photosensitive element, the image-receiving layer being capable of mordanting or otherwise fixing the image-providing material. The image-receiving layer retains the transferred image for viewing, and in some diffusion transfer products, the image is viewed in the image-receiving layer after separation from the photosensitive element (e.g., "peel apart" film units), while in other products, such separation is not required (e.g., "integral" film units).
Image-dye providing materials which may be employed in diffusion transfer processes generally may be characterized as initially soluble or diffusible in the processing composition but selectively rendered nondiffusible in an imagewise pattern as a function of development, or as initially insoluble or nondiffusible in the processing composition but selectively rendered diffusible in an imagewise pattern as a function of development. These materials may contain complete dyes or dye intermediates, e.g., color couplers. A particularly useful class of image-dye providing materials for diffusion transfer processes are dye developers. These compounds contain, in the same molecule, both the chromophoric system of a dye and also a group adapted to develop exposed photographic silver halide.
Although the principles and subject matter presented herein are readily adaptable to other diffusion transfer products and processes, in order to facilitate the presentation herein, representative diffusion transfer products and processes utilizing dye developer image-dye providing materials have been selected to be discussed in more specific detail below.
Multicolor images in a representative diffusion transfer system can be obtained by arranging a photosensitive element with at least two silver halide layers selectively sensitized to different regions of the spectrum. Such a system is shown, for example, in U.S. Pat. No. 2,983,606. To accomplish subtractive color photography, associated with each silver halide layer is a dye developer featuring an absorption that is substantially complementary in color to the color of the light recorded in the contiguous silver halide layer. The most commonly employed arrangement of this type includes three monochrome units--a blue-sensitive silver halide layer overlying a yellow dye developer, a green-sensitive silver halide layer overlying a magenta dye developer, and a red-sensitive silver halide layer overlying a cyan dye developer.
Such an exposed photosensitive dement is processed using an aqueous alkaline processing composition containing an alkali, such as potassium hydroxide. The processing composition penetrates the layers of the negative element and dissolves the dye developer compounds by ionizing the developer groups. In each silver halide layer, where the silver halide has been exposed and developed, the dye developer becomes insoluble or at least substantially immobile. In unexposed regions of silver halide, the solubilized dye developer diffuses through the overlying layers to an image-receiving layer to form a positive multicolor image.
The photographic transfer dye image frequently retains substantial amounts of photographic reagent, particularly alkali, with which it has been processed, even though the layer of processing composition is caused to adhere to and remain with the photosensitive layer. The presence of those residual reagents may adversely effect the quality and stability of the image, particularly in the presence of significant quantities of alkali. For example, if the receiving layer is highly alkaline, oxidation by atmospheric oxygen of unreacted developing agent or other component of the processing composition is quite likely to occur, and such reactions or subsequent reactions may impart a stain or otherwise discolor the transfer image, particularly the highlights thereof.
Since the processing compositions employed in diffusion transfer products and processes are highly alkaline (i.e., pH&gt;12), after processing has been allowed to proceed for a predetermined period of time, it is desirable to treat the image-receiving element with a solution which is effective to remove (as by a washing action), neutralize or otherwise render such processing agents relatively innocuous. A neutralizing layer, typically a nondiffusible polymeric acid-reacting agent, has been employed in previous diffusion transfer film units to lower the pH from a first (high) pH of the processing composition to a predetermined second (lower) pH. See e.g., U.S. Pat. No. 3,362,819, issued to E. H. Land on Jan. 9, 1968. In this regard, the polymeric acid, acting as an ion exchanger, forms an immobile polymeric salt with the alkali cation and returns water in place of alkali. Capture of alkali by the polymer molecule prevents deposition of salts on the print surface. The dye developers thus become immobile and inactive as the pH of the system is reduced. As examples of other previously :patented neutralizing layers, in addition to those disclosed in the aforementioned U.S. Pat. No. 3,362,819, mention may be made of those disclosed in the following U.S. patents: Bedell U.S. Pat. No. 3,765,885; Sahatjian et at. U.S. Pat. No. 3,819,371; Haas U.S. Pat. No. 3,833,367; Taylor U.S. Pat. No. 3,756,815 and Schlein U.S. Pat. No. 3,756,815.
In Land '819, as with most other previous systems, the polymeric acid neutralization layer was typically deposited onto a dimensionally stable substrate by means of an organic solvent, such as acetone, methyl ethyl ketone, methanol, ethyl acetate, butyl acetate, and mixtures thereof. While organic solvent-based neutralization layers continue to be used, interest has expanded toward the development of an aqueous-based (i.e., water soluble) neutralization layer.
The use of water-soluble ingredients in the neutralizing layer affords several distinct advantages over systems employing water-insoluble solvent-coated components. Apart from the readily apparent advantage in avoiding the use of organic solvents, water-soluble compositions may utilize shorter acid molecules with lower equivalent weights, thereby permitting one to employ thinner coatings. Advantage is gained, since the neutralization layer, in certain diffusion transfer products, should be as thin and clear as possible in order to minimize distortion and/or to obtain the most optimal resolution.
Despite advantage, previous efforts to provide aqueous-based neutralization layers, resulted almost invariably in film units with an unacceptable and unresolvable propensity to curl. See, Schlein '815. Curling was most evident prior to use, making subsequent processing of the film unit for image formation difficult. Curling also manifested in the finished print, which was and continues to be undesirable from an aesthetic standpoint. To the extent that it occurred, gains in the control of cud in aqueous-based systems of previous efforts came at the sacrifice of other aspects of sensitometric performance and/or the unacceptable increase of manufacturing costs.
In view of the above, there has been a long-felt and yet unsatisfactorily resolved need for a composition useful in making a curl-resistant aqueous-based neutralization system having desirable, functional characteristics, such as adequate acid neutralization capabilities, thinness, and clarity. Related need also exists for viable and effective methods for making the compositions used for such neutralization layers.